the soil: ozonized oxygen changes them in the same way. The earth
and calcareous rocks of caves, penetrated by the air, slowly produce
saltpetre, and before the theory of the action was understood,
artificial imitation of natural conditions enabled us to manufacture
saltpetre. Animal remains, stratified with porous earth or the sweepings
of cities, and disposed in long heaps or walls, protected from rain, but
exposed to the prevailing winds, soon form nitrous salts, and a large
space covered with these deposits carefully tended forms a saltpetre
plantation. France, Prussia, Sweden, Switzerland, and other countries,
have been supplied with saltpetre from similar artificial arrangements.

But the atmosphere is washed most thoroughly by the rains falling in and
near tropical countries, and the changes there are most rapid, so that
the production of saltpetre, favored by moisture and hot winds, attains
its highest limit in parts of India and the bordering countries.

During the prevalence of dry winds, the earth in many districts of India
becomes frosted over with nitrous efflorescences, and the great quantity
shipped from the commercial ports, and that consumed in China, is thus a
natural production of that region. The increased amount due to tropical
influences will be seen in the instances here given of the produce from
the rich earths of different countries:--

_Natural_.

France, Church of Mousseau, 5-3/8 per cent.
" Cavern of Fouquières, 3-1/2 "
U. States, Tennessee, dirt of caves, 0.86 "
Ceylon, Cave of Memoora, 3-1/10 "
Upper Bengal, Tirhoot, earth simply, 1-6/10 "